an important move, attacking both the knight and the pawn on c5.
17. a1b1 White decides to make an improving move before capturing on c5, since Black cannot take on d4. But it's not clear whether 17.Rab1 is the most useful move. 17... f8e8  18. d4c5  18... d5d4 A very difficult choice. 19. c3c4 Another tough choice, now for White. This move allows Black to equalize.